This invention relates to thermomechanical vacuum regulators which are adapted for use in automotive automatic temperature control systems. The invention more particularly relates to a vacuum regulator which produces a vacuum in accordance with the temperature of an air stream through the use of a bimetal strip.
As well known to those skilled in the art, it has been common to mix heated air and cool air in such proportions in an automotive vehicle with the help of an air mix damper or blend door so that the mixture of such air is at the temperature set by the vehicle operator. It has also been common to use a vacuum powered motor to position the blend door to produce the desired mixture of air. The means for controlling the vacuum motor have comprised vacuum regulators, certain of which operate in accordance with thermomechanical principles whereby a bimetal element suspended in the air whose temperature is to be controlled in turn controls the degree of vacuum communicated to the vacuum motor. These vacuum regulators have included a means for communicating ambient air to a chamber in which the controlled vacuum is produced and which in turn is freely communicated with the vacuum motor. The vacuum regulators have also included means which communicate a source of vacuum, suitably the engine manifold, with the chamber. Feedback means are also known, usually in the form of a floating diaphragm which cooperates with the above mentioned means, to permit the regulated vacuum to be fed back into the vacuum regulator servo system. In essence, the conversion between temperature of the bimetal element and the vacuum output from the vacuum regulator is achieved by balancing the forces between the vacuum feedback diaphragm of the regulator and the bimetal element to control the vacuum and vent means. In a typical vacuum regulator of this prior art type, the vent and vacuum means, usually valves, are arranged colinearly with the line of action through which the feedback diaphragm and bimetal element forces operate. The inherent difficulty with this type of system is that the forces tending to produce the regulated vacuum decrease asymptotically as the vacuum reaches its proper level since the forces that position the diaphragm and the valving are colinear. In addition, the bimetal element is under stress from the load of the feedback diaphragm for long periods of time and thus overall accuracy of the bimetal is affected with time. Another difficulty with this prior art system is that the response time of the system after an unbalance of the system occurs is limited by the amount that the valves are open since the bimetal element must divide its output force and motion between the feedback diaphragm and the opening of the valves.
The present invention overcomes the disadvantages found in the use of the prior art thermomechanical vacuum regulators using bimetal elements by the use of position sensing of the bimetal element and by positioning a servo follower valve in response to the position of the bimetal element. This mode of operation presents practically no force reaction to the bimetal element and allows it to stroke in accordance with its temperature. A vacuum regulator built in accordance with the principles of this invention will provide wider valve openings and thus faster correction of an unbalanced system.
One embodiment of the thermomechanical vacuum regulator to be described is a zero leakage device which will provide a vacuum output as a function of the ambient temperature of an active bimetal element. In other words, when the vacuum regulator is in a condition of equilibrium vacuum and vent valves are closed so that vacuum is conserved. The bimetal element is designed as a simple beam pivoted at the center. Motion of the beam is constrained at one end by a support which can be adjusted in accordance with the desired temperature. The opposite end of the bimetal element is in contact with a whiffle tree beam which is free to move as the bimetal element deflects as a function of temperature or temperature setting. To minimize system hysteresis, the bimetal element and whiffle tree beam are preloaded by a helical compression spring which maintains the load on the bimetal element and whiffle tree beam attachment point in one direction. Hysteresis as a result of play at the pivots is thereby eliminated. The helical compression spring is suitably designed with a relatively low spring rate so that the force applied to the whiffle tree beam is relatively constant throughout the normally expected deflection range. Two needle valves are attached to the whiffle tree beam with unidirectional spherical bearings. The needle valves are maintained in zero play connection to the whiffle tree beam by magnetic attraction, the whiffle tree beam being suitably of steel and the needle valves being magnetized steel. The vacuum valve is located at the end of the whiffle tree beam remote from the attachment point of the bimetal of the whiffle tree beam. The vacuum valve controls the input vacuum flow into a regulated vacuum chamber. The vacuum valve seat is fixed. The vent valve is located centrally on the whiffle tree beam with the vent valve seat being positioned on a feedback piston comprised of a flexible diaphragm which forms one wall of the controlled vacuum chamber. There is thus present on one side of the feedback piston the controlled vacuum and on the other side ambient air. The location of the piston is thus determined by the controlled vacuum and a feedback leaf spring which normally biases the feedback piston upward.
The bimetal element is arranged in a stream of air whose temperature is to be controlled so that the resultant controlled vacuum is related to the temperature of the air stream and an adjustable force which can be applied to the bimetal element and which is related to a selected desired temperature.
When the vacuum regulator is in equilibrium the feedback piston assumes a position such that both the vent valve and the vacuum valve are closed. In this equilibrium condition there is no air bled into the vacuum regulator, hence there is no waste of engine vacuum.
In an alternative embodiment, one of the vacuum or vent valves is replaced by an orifice whereby engine vacuum or ambient air respectively is continuously communicated with the regulated vacuum chamber and the valve controlled to regulate the vacuum therein.
It is an object of this invention to provide a vacuum regulator which operates on thermomechanical principles.
It is another object of this invention to provide a vacuum regulator of the type described which is particularly adapted for use in an automotive automatic temperature control system.
It is a further object of this invention to provide a thermomechanical vacuum regulator, particularly for use in automotive vehicles which provides a controlled vacuum for controlling an automatic temperature control system in response to actual temperature and desired temperature.
It is another object of this invention to provide a vacuum regulator of the type described which includes a thermally responsive mechanical element which acts through valve means for controlling vacuum within a chamber and wherein the forces exerted by said mechanical element are minimal.